The present invention relates to a method and apparatus for dispensing compressed gas. More particularly, the present invention is directed to a method and apparatus useful for transferring a compressed gas from a refueling station into one or more storage tanks.
Because of the interrelationship between the temperature, pressure, and density of gases, the amount of hydrogen, H2, (or compressed natural gas (CNG)) that can safely be introduced into a storage tank, such as a vehicle storage tank, during refueling necessarily depends upon factors such as the volume, design pressure, and temperature of the tank, and the temperature and pressure of the compressed gas inside the tank. Industry convention sets the pressure rating for H2 fuel tanks at the standard temperature of 15 degrees Celsius, so nominal pressure ratings such as 250 bar (25 MPa), 350 bar (35 MPa), 500 bar (50 MPa) and 700 bar (70 MPa), correspond to an internal gas temperature of 15 degrees Celsius. During rapid refueling of hydrogen, the internal tank temperature will typically rise about 50 degrees Celsius due to adiabatic compression of the gas and the reverse Joule-Thompson effect. After the tank is filled, the temperature and pressure inside the tank will decrease as the gas cools. Wide variations in ambient temperature above or below the standard condition of 15 degrees Celsius can also have a significant effect on the indicated pressure inside the tank during and after refueling.
As referred to herein, a compressed gas includes pressurized gas and supercritical fluids. A pressurized gas is a fluid below its critical pressure and below its critical temperature. A supercritical fluid is a fluid above either its critical pressure or its critical temperature.
It is desirable to have a method for dispensing compressed gas to a receiving vessel without the problem or risk of overfilling, so that an no time would the rated pressure in the tank be exceeded.